When a device records an image, light passes through a lens and lands on a photodetector—a light-sensitive material like the sensor in a digital camera. However, a lens bends the light and curves the focusing plane. In a digital camera, the point where the focusing plane meets the flat sensor will be in focus, but the image becomes more distorted the farther it is from that focus point.
High-end digital cameras correct this problem by incorporating multiple panes of glass to refract light and flatten the focusing plane. However, such lens systems are large, bulky and expensive.